The invention relates to an electric starter for an internal combustion engine.
In luxury cars, so-called tip starters are preferably used, which automatically perform the starting operation for the internal combustion engine by means of a starting pulse. The result is a decoupling between the selection of starting and the starting event itself. In tip-starters, the starting event is thus controlled electronically and is ended at the earliest possible moment, for instance once the engine is at a predeterminable minimum rpm. Since the starting event is not successful in all cases, provision must be made to assure that the starting event will not be continued indefinitely, both to protect the starter battery and to avoid a thermal overload on the electric starter. It has been found that in electric starters, the hottest points, with the greatest potential risk, are the carbon brushes and the commutator.
To make a thermal monitoring protection for such starters feasible, it is known in the prior art to dispose a bimetallic switch in the region of or next to the so-called terminal 45 (positive starter terminal). Disposing the bimetallic switch in the region of the critical (hottest) points, however, is either very difficult or even impossible in engineering terms. The bimetallic switches must accordingly be designed such that they react, i.e. switch, at a temperature at which it is assumed that a maximum possible limit operating temperature is occurring at the highest-risk points of the starter.
Such bimetallic switches are indeed simple in design and in their principle of operation, but they are relatively large. Moreover, their switching performance involves to severe hysteresis. Besides, the activation and deactivation temperature is highly subject to variation. Since the bimetallic switch cannot be made with the requisite low impedance necessary for the main starter current, the choice is made to have the bimetallic switch trigger a relay, which switches the main starter current. This arrangement means an additional plug connection in the relay circuit and a double line between the relay and the bimetallic switch. Thus it contributes to increasing the resistance in the relay circuit.
The electric starter for an internal combustion engine offers the advantage over the prior art that because a virtual operating temperature of the starter is ascertained, sensor costs and installation space, lines, and an increase in resistance in relay circuit no longer occur. Moreover, turning the electric starter on and off does not involve hysteresis, and the activation and deactivation temperature can be specific precisely. Finally, the electric starter of the invention offers the advantage that the problems of the difference in temperature response between the sensor point and the actual monitoring point (brushes, commutator) no longer arise. These advantages are attained in that a virtual operating temperature of the starter is ascertained, and it is provided that this virtual operating temperature is ascertained as a function of at least one operating parameter that affects the operating temperature of the starter. It has been found that when a virtual operating temperature is ascertained as a function of an operating parameter of the starter, a high degree of agreement with actual temperatures occurring at the various components is obtained. In this respect, the electric starter of the invention thus offers the advantage thatxe2x80x94without direct measurementxe2x80x94the temperature range of interest can be detected very precisely, and the temperature of the electric starter can be ascertained precisely.
In a further feature of the invention, it is provided that in operation of the starter, that is, as the operating temperature of the starter rises, the virtual operating temperature is ascertained at least as a function of the starter current Primarily it is the operating current of the starter that causes the heating of the starter. That is, if this operating parameter is taken into account in ascertaining the virtual operating temperature, then a heating model can be made that very precisely reflects again the operating temperature of the starter as a function of the ON time of the starter.
Preferably, when the operating temperature is falling, the OFF time of the starter is taken into account. That is, the virtual operating temperature is affected as a function of the OFF time. Hence if the starter is not in operation, then from the OFF timexe2x80x94on the basis of the previously ascertained, higher virtual operating temperaturexe2x80x94the cooling temperature or temperature gradient is ascertained.
In a preferred embodiment of the electric starter, it is provided that the virtual operating temperature is ascertained as a function of a reference temperature, in particular the ambient temperature of the starter. Thus it can be provided that when the starter is in operation, the rising virtual operating temperature is ascertained on the basis of the ambient temperature. Conversely, in the cooling-down phase, it can be stated that the virtual operating temperature cannot drop below the ambient temperature.
It is preferably provided that the device has a means for detecting the ambient temperature. It is thus possible to perform a calibration between the virtual operating temperature and the ambient temperature, since the detection means ascertains the actual ambient temperature. A reference can thus be made between the virtual operating temperature and an actually measured temperature, especially the ambient temperature or the reference temperature.
One exemplary embodiment is distinguished in that the virtual operating temperature is ascertained as a function of the current ratio (i/i0)b. That is, a standardized starter current is taken into account, where i is the actual starter current, i0 takes a reference current into account. The exponent b can be assumed to be a starter-specific parameter. Because the virtual operating temperature is ascertained as the function of the current ratio, a reference can also be established for whether the starter is heating up very quickly or not so much. If because of temperature factors, for instance, the engine is subjected to an increased torque in the starting event, usually the consequence is an increased starter current. The starter would accordingly heat up much faster. This is taken into account in ascertaining the virtual operating temperature as a function of the current ratio.
To make it possible to graph the cooling down of the starter very precisely, it is preferably provided that during the OFF time of the starter, increased cooling of the starter is assumed, if the virtual operating temperature is especially high. In other words, if a high temperature difference exists between the virtual operating temperature and the reference temperature, in particular the ambient temperature, then the starter can dissipate its heat to the environment relatively quickly. In the upper temperature range, the cooling down is thus speeded up. This is taken into account with the provision according to the invention.
If the OFF times last relatively long, it can be provided that a reduction in the cooling gradient of the virtual operating temperature is assumed. That is, as the virtual operating temperature falls, it approaches the ambient temperature more and more slowly. This influence is accordingly taken into account in the ascertainment of the virtual operating temperature.
In a preferred exemplary embodiment, it can be provided that as a function of the OFF time of the starter and of its instantaneous virtual operating temperature, the virtual operating temperature is set equal to the ambient temperature. For instance, if the starter is out of operation for a relatively long period of time, and if the model calculation assumes that the starter has already cooled down completely, but the virtual operating temperature is above or below the ambient temperature, thenxe2x80x94to make it possible to start the system again from a reference pointxe2x80x94the virtual operating temperature is set equal to the ambient temperature. It is thus assumed here that after a certain OFF time, the starter has the same temperature as the ambient temperature or reference temperature. This assures that any error that may have been incorporated into the determination of the virtual operating temperature is corrected at defined time intervals.
It can also be provided that as a function of the cooling gradient, the virtual operating temperature is set equal to the ambient temperature or reference temperature. If the virtual operating temperature decreases only very slightly over time, then the virtual operating temperature can be reset to the ambient temperature, since it can be assumed that the actual operating temperature of the starter is virtually identical to the ambient temperature.
Alternatively, it can also be provided that as a function of the OFF time and/or of the cooling gradient, the virtual operating temperature is adapted in stages to the reference temperature or ambient temperature.
Further advantageous features will become apparent from the dependent claims.